The invention relates to the field of fibre web dewatering. In particular, the invention relates to a long pressing zone press for the treatment of a fibre web.
The dewatering of a fibre web is usually carried out by means of roller presses, by leading the web via a press zone formed by two rolls, i.e. through a press nip. The web runs through the nip between press felts and the felts carry away the water squeezed out of the web.
In a nip formed by two rolls, the greatest pressing force is reached as a narrow peak in the middle of the nip. This is disadvantageous both to the dewatering process and to the service life of the felts, because at high velocities, the pressing is of a very short duration and great stress is put on the felts. Therefore, different kinds of so-called long nip presses have been constructed wherein one roll can be substituted by a concave counterpart, by a press shoe. On the shoe side, the rotary motion of the roll is usually matched by a watertight, endless loop of fabric, a press belt that glides on the surface of the lubricated counterpart following its shape. The press belt is supported by separate rolls, or alternatively, the shoe support is given a shape that enables a short belt to glide around it, lubrication being provided on the inside of the endless loop formed by the belt. By the use of shoe presses, press nips whose effective length can be approximately 250-310 mm, depending on the size of the roll, are achieved. In addition to roll/shoe combinations, the patent documents mentioned later disclose presses composed of two opposite shoes.
Patent application CH5152/86 discloses a press shoe. On its surface, under the press belt, cavities are provided that serve as hydrostatic pressure chambers when hydraulic fluid is led into them through channels in the shoe. In addition to pressure, a stepped heat treatment for the web is achieved by individually regulating the temperature of the hydraulic fluid in the separate chambers. Patent application FI 896163 also discloses a press shoe having a plurality of pockets on its sliding surface for feeding an individually pressurised lubricant, the pressure profile being adjustable in the nip zone for desired pressing and web speed conditions. Swedish patent application SE 9103823-2 discloses a press shoe provided with a pressure pocket, wherein the depth profile of the pressure pocket results in a combination of hydrostatic and hydrodynamic effects for obtaining a desired pressing force profile.
An object of the present invention is to provide a straight forward long nip press structure useable for the dewatering of a fiber web. The shoeless nip press of the present invention includes a roll and a concave counterpart. A hydraulic pressure chamber is provided on the surface of the counterpart, under a gliding press belt. No press shoe in the usual sense of the word is employed, as a result of which no hydraulic cylinder control systems, which are typical of she presses, for adjusting the tilt and the radial position of a shoe are needed.
Because of the simple structure, the sealing of the edges of the hydraulic pressure chamber of the counterpart remains a problem. According to the present invention, the press belt is tightened by pressurising the space on the inside of the press belt in the counterpart. This pressure will hereinafter be referred to as belt pressure. The belt pressure, which is applied to the inner surface of the belt from the direction of the counterpart, is proportional to the pressure of the pressure chamber. When the ratio of the radius of curvature of the pressure chamber edge to the radius of the general path of the belt corresponds to the ratio of the belt pressure to the pressure of the pressure chamber, no oil leakage occurs from the pressure chamber. When the ratio of said pressures is maintained at at a value appropriately lower than that of the ratio of said radii to each other, a sufficient amount of oil escapes from the pressure chamber to lubricate the sliding surfaces between the belt and the counterpart.
The structure of the press is thus considerably simple but provides sufficient means to regulate the compression pressure in the nip zone and, consequently, to influence the properties of the pulp web leaving the press. The edges of the pressure chamber are located outside the nip zone. As the pressure of the chamber forces the belt to follow the surface of the roll, the nip length can be adjusted by adjusting the distance between the roll and the counterpart.
The belt pressure can be generated by means of a fluid, preferably oil; or gas, preferably air. Preferably, the belt is made from fiber reinforced rubber.